1. Field of the Invention
The present invention relates to an encryption device and a decrypting device for protecting copyrights when transmitting digital productions via a recording medium or a transmission medium. More specifically, the present invention relates to a protection technique that is to protect against an attack enacted by is a replacement of a Certificate Revocation List (CRL) specifying a revoked public key certificate.
2. Description of the Related Art
When a digital production is transmitted from a (first) device to another (second) device, prior to the transmission, a computer authentication is conducted to avoid a copyright infringement by an authorized obtainment. The first computer is to authenticate the second computer. In other words, the first computer makes sure that the second computer is a qualified computer to transmit.
For example, a first computer sends a random number to a second computer, then the second computer encrypts the random number with its own secret key (i.e., digital signature) and transmits it to the first computer. Finally, the first computer verifies the transmitted encrypted text (or the digital signature) using the second computer's public key.
However, the authentication using the public key encryption should be based on the condition that the public key itself is not revoked.
Therefore, in recent years, a “public key certificate” for proving that a public key is a qualified key for each user, is issued from an organization or a company called “certificate authority” (CA). Among those issued public key certificates, there are certificates for the users who have a secret key that is expired or stolen, or that have done something illegal. For nullifying those certificates (or notifying other users that those certificates are nullified), a Certificate Revocation List (hereinafter referred to as a “CRL”, a “public key certificate revocation list” or an “revocation list”), an information list for specifying the revoked public key certificate is issued.
Accordingly, when authenticating a communication partner with the partner's public key, a public key certificate is obtained from the communication partner, and upon confirmed that the obtained public key certificate is not listed on the CRL, and then the above-mentioned authentication processing is executed so as to avoid transmitting a valuable digital production to an unauthorized communication partner.
There are devices and systems (refer to Japanese patent NO. 3199119) in which key checking is conducted with only referring to the public key certificate, however, such devices and systems cannot cover when there are certificates for the users who have a secret key that is expired or stolen, or have done something illegal, as stated above.
However, it is not possible for every computer to obtain the qualified CRL and check the validity of the public key certificate of the communication partner. As a result, unauthorized use is conducted.
For example, a device, such as a DVD drive device which plays back a DVD (digital Video/Versatile Disc), on which digital works (i.e., movies) are recorded, obtains the qualified CRL via a DVD and reads out the latest CRL from the DVD, and then authenticates the communication partner computer (a computer that operates an integrated playback processing circuit or playback software) with reference to the CRL. In the process of reading out the CRL, there is a possibility that the CRL could be replaced with the old one.
As a result, although a computer is listed on the qualified (i.e., the latest) CRL as a revoked computer, it may be possible for the revoked computer to be transmitted a digital production illegally with a revoked public key that is not listed on the replaced old CRL yet.
Also, when a computer which has already held a CRL obtains a new CRL, it is necessary to compare the two lists to figure out which is the latest, then holds only the latest one, that is, it is necessary to verify accurately which lists should be held.
Accordingly, the first object of the present invention is, in the light of the above-mentioned problem, to provide an encryption device, a decrypting device, a secret key generation device, a copyright protection system and a cipher communication device that can defend from an attack enacted by a replacement of a CRL, and as a result, transmit a digital production safely.
And the second object of the present invention is to provide a cipher communication device that can specify the latest CRL accurately when a new CRL is obtained, and hold only the latest list in place of the old one.